I want to do a full scan of windows defender automatically

Protecting Devices with Windows Security, as described in

Windows Defender has a scan task registered with the Task Scheduler, so you only need to set the trigger to run at the desired time.

"It doesn't work," but if you can't do it this way, please explain what doesn't work.

I don't know the specific steps or specifications, but I think the following pages will be helpful.

The most suitable pages are as follows.
Configure a scheduled quick scan or a full Windows Defender antivirus scan

There are several other brief descriptions.
Windows Defender
on the command line (CUI) This is Windows 7, but Tip: Run (automate) Windows Defender on the command line
Also, the text is strange because of the automatic translation, but the Windows 10 documentation
Configure and use the mpcmdrun.exe command-line tool to manage Windows Defender virus protection

The first pages for Windows 10 are as follows, so please look into the following documents:
Windows Defender Advanced Threat Protection